The present invention relates to methods for making sputter targets for magnetron sputtering, sputter targets made by the methods, and methods of sputtering using such targets. More particularly, the invention relates to the manufacture of sputter targets using nickel-silicon alloys and to targets manufactured thereby.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates such as semiconductor wafers. Basically, a cathode assembly including a sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form the thin layer or film.
In so-called magnetron sputtering, one or more magnets are positioned behind the cathode assembly to generate a magnetic field. Magnetic fields generally can be represented as a series of flux lines, with the density of such flux lines passing through a given area, referred to as the xe2x80x9cmagnetic flux density,xe2x80x9d corresponding to the strength of the field. In a magnetron sputtering apparatus, the magnets form arch-shaped flux lines which penetrate the target and serve to trap electrons in annular regions adjacent the sputtering surface. The increased concentrations of electrons in the annular regions adjacent the sputtering surface promote the ionization of the inert gas in those regions and increase the frequency with which the gas ions strike the sputtering surface beneath those regions.
Nickel is commonly used in physical vapor deposition (xe2x80x9cPVDxe2x80x9d) processes for forming nickel silicide films by means of the reaction of deposited nickel with a silicon substrate. Yet, while magnetron sputtering methods have improved the efficiency of sputtering many target materials, such methods are less effective in sputtering xe2x80x9cferromagneticxe2x80x9d metals such as nickel. It has proven difficult to generate a sufficiently strong magnetic field to penetrate a nickel sputter target to efficiently trap electrons in the annular regions adjacent the sputtering surface of the target.
In order to provide a background for the present invention, certain aspects of the magnetic behavior of metals will be briefly described.
The magnetic flux density vector within a metal body generally differs from the magnetic flux density external to the body. Typically, the component xe2x80x9cBxe2x80x9d of the magnetic flux density along a given direction in space within a metal body may be expressed in accordance with the relationship B=xcexc0 (H+M), where xe2x80x9cxcexc0xe2x80x9d is a constant referred to as the magnetic permeability of empty space; xe2x80x9cHxe2x80x9d is the corresponding component of the so-called xe2x80x9cmagnetic field intensityxe2x80x9d vector; and xe2x80x9cMxe2x80x9d is the corresponding component of the so-called xe2x80x9cmagnetizationxe2x80x9d vector. (Note that positive and negative values of the components of the magnetic flux density, the magnetic field intensity and the magnetization represent opposite directions in space, respectively.)
The magnetic field intensity may be thought of as the contribution to the internal magnetic flux density due to the penetration of the external magnetic field into the metallic body. The magnetization may be thought of as the contribution to the internal magnetic flux density due to the alignment of magnetic fields generated primarily by the electrons within the metal.
In xe2x80x9cparamagneticxe2x80x9d materials, the magnetic fields generated within the metal tend to align so as to increase the magnetic flux density within the metal. Furthermore, the magnetic fields generated within a paramagnetic metal do not strongly interact and cannot stabilize the alignment of the magnetic fields generated within the metal, so that the paramagnetic metal is incapable of sustaining any residual magnetic field once the external magnetic field is removed. Thus, for many paramagnetic metals and at a constant temperature, the xe2x80x9cmagnetization curve,xe2x80x9d which relates the magnetic flux density to the magnetic field strength within the metal, is linear and independent of the manner in which the external magnetic field is applied.
In a xe2x80x9cferromagneticxe2x80x9d metal such as nickel, the magnetic fields generated within the metal do interact sufficiently for the metal to retain a residual magnetic field when the external field is removed. Below a xe2x80x9cCurie temperaturexe2x80x9d characteristic of a ferromagnetic metal, the metal must be placed in an external magnetic field directed oppositely to the residual field in the metal in order to dissipate the residual field.
At any constant temperature below the Curie temperature, the relationship between the magnetic flux density and the magnetic field intensity in the metal differs depending on how the external magnetic field has varied over time. For example, if a ferromagnetic metal is magnetized to its maximum, or xe2x80x9csaturation,xe2x80x9d flux density in one direction in space and then the external magnetic field is slowly reversed to the opposite direction, the magnetic flux density within the metal will decrease as a function of the magnetic field intensity along a first path until the magnetic flux within the metal reaches the negative of the saturation value. If the external field is again reversed so as to remagnetize the metal in the original direction, the magnetic flux density within the metal will increase as a function of the magnetic field intensity along a second path which differs from the first path in relation to the reversal of the residual magnetic field. The shape of the resulting dual-path magnetization curve, which is referred to as a xe2x80x9chysteresis loop,xe2x80x9d is characteristic of ferromagnetic behavior.
When a ferromagnetic metal is surrounded by a gas in the presence of a magnetic field, the ferromagnetic metal tends to xe2x80x9cattractxe2x80x9d the flux lines of the magnetic field away from the surrounding gas into itself. This prevents the flux lines from penetrating the ferromagnetic metal and extending through to the surrounding gas. While paramagnetic metals may xe2x80x9cattractxe2x80x9d some flux lines of an external magnetic field, they do so to a far lesser degree than do ferromagnetic materials.
Above their Curie temperatures, nominally ferromagnetic metals behave in a manner similar to paramagnetic materials. In particular, nominally ferromagnetic metals tend to xe2x80x9cattractxe2x80x9d far less of the flux of an external magnetic field into themselves above their Curie temperatures than below.
Thus, without wishing to be bound by any theory of operation, it is believed that a nickel sputter target placed in the magnetic field of a magnetron sputtering device tends to xe2x80x9cattractxe2x80x9d the flux of the magnetic field into itself. This prevents the magnetic flux from penetrating through the target, thereby reducing the efficiency of the magnetron sputtering process.
Typically, only thin nickel targets of about 0.12 inch (3 mm) or less could be used in magnetron sputtering processes due to the ferromagnetic character of nickel. This increases the difficulty and cost of sputtering nickel, since it is necessary to replace the sputter targets at frequent intervals.
Meckel U.S. Pat. No. 4,229,678 sought to overcome this problem by heating the target material to its Curie temperature and magnetron sputtering the material while in such a state of reduced magnetization. Meckel further proposed a magnetic target plate structured to facilitate heating of the plate to its Curie temperature by the thermal energy inherent in the sputtering process. One drawback to this proposed method was the increased cost inherent in providing for the heating of the target as well as providing for the stability of the cathode assembly at increased temperatures.
The problem of magnetron sputtering nickel has been addressed in the specialty media industry by alloying the nickel with another transition metal such as vanadium. At about 12 at .% vanadium, the alloy ceases to behave ferromagnetically. Alloys of nickel with other transition metals such as chromium, molybdenum and titanium have shown a loss of ferromagnetic behavior at compositions under 15 at .%. Adopting a similar approach, Wilson U.S. Pat. No. 4,094,761 proposed alloying nickel with copper, platinum or aluminum to produce an alloy having a Curie temperature below the sputtering temperature. Unfortunately, all of these methods share the drawback that the metals alloyed with the nickel constitute impurities when the sputter target is used in a nickel silicidation process.
Therefore, there remains need in the art for a method for making a nickel sputter target which is compatible with magnetron sputtering processes.
These and other objects of the invention are met by a method for making a nickel/silicon sputter target including the step of blending molten nickel with sufficient molten silicon so that the blend may be cast to form an alloy containing no less than 4.5 wt % silicon, preferably about 4.5-50 wt % Si. The cast ingot is then shaped by rolling it to form a plate having a desired thickness and then the rolled plate is machined to form the desired target shape. The sputter target so formed is capable of use in a conventional magnetron sputter process; that is, it can be positioned near a cathode in cathodic sputtering operations, in the presence of an electric potential difference and a magnetic field so as to induce sputtering of nickel ion from the sputter target onto the substrate. However, these targets can be made thicker than conventional Ni targets so that they may be used for longer sputtering times without replacement.
Nickel-silicon alloy sputter targets in accordance with the invention have been found to exhibit sufficiently low Curie temperatures that their behavior at conventional sputtering temperatures is thought paramagnetic rather than ferromagnetic. Thus, the magnetizations of targets having thicknesses as large as 0.5 inch (1.3 cm) are sufficiently low that the targets may be used in conventional magnetron sputtering processes. Furthermore, the nickel/silicon alloy does not introduce any impurities when the target is used for nickel silicidation.
In addition, it has been found that rolling the ingot formed from casting the nickel-silicon alloy before machining the target promotes the deposition of a uniform layer of nickel silicide during the sputtering process.